The APA Publicaton Manual devotes two entire sections to crediting electronic sources. Refer to Section 6.31 Electronic Sources and Locator Information and 6.32 Providing Publication Data for Electronic Sources, pages 189-192. The link below is a PDF version of section 6.31 and explains URLs and the DOI system.
Online articles follow the same guidelines for printed articles. Include all information the online host makes available, including an issue number in parentheses.
Journal article with DOI
Author, A. A., & Author, B. B. (Year). Title of article. Title of Online Periodical, volume
number (issue number if available). doi:xx.xxxxxxxxxx
Journal article with no DOI
Author, A. A., & Author, B. B. (Year). Title of article. Title of Online Periodical, volume
number (issue number if available). Retrieved from http://www.someaddress.com/full/url/
When referencing material obtained from an online database (such as a database in the library like EBSCO, ProQuest, etc.), provide appropriate print citation information (formatted just like a "normal" print citation would be for that type of work). This will allow people to retrieve the print version if they do not have access to the database from which you retrieved the article.
You can also include the item number or accession number in parentheses at the end, but the APA manual says that this is not required.
For electronic versions based on print sources (as in PDF), give inclusive page numers for the article cited.
Provide the DOI, if one has been assigned. Use this format for the DOI in references: dio:xxxxxx
When no DOI has been assigned to the content, provide the home page URL for the database from which you found the journal, book, or report. Use this format: Retrieved from http://www.xxxxxxxx
It is not necessary to include database information. If the article is difficult to locate, then you can provide database information, (JSTOR, etc.)
Do not include retrieval dates unless the source material may change over time.